That's because you have the QuickTime DivX 5.something plug-in installed. It's mostly Quicktime 7 compatible, but causes a few glitches, the double help menu being among them.
They're apparently still "working on" a Mac version of the v6 plugin, which will presumably be compatible with QT7 (and therefore Tiger) if and when it is released. You'd think after several months of developer betas and then several more months of release that they could get the plugin polished up, but I guess it's more involved than it seems.